Screen used downhole typically involved welding the screen material to a base pipe at opposed ends to retain the layers together. Typical of such assembly techniques are U.S. Pat. Nos. 3,908,256; 3,958,634; 4,284,138 and 4,428,423. There are drawbacks to the welding technique of attaching a filter layer to a base pipe below it. One hazard is that the weld or welds could give out under conditions creating stress between the base pipe and the filtration layer. The base pipe could be in tension to such an extent from forces applied to it from above or from weight of tools and pipe hanging below that a weld failure could occur. When exotic materials are used there are added risks of corrosion at the welds to the base pipe. Since the filtration layer could become obstructed in use, concerns of collapse resistance of the filtration layer could arise in that the only secure connection to the base pipe is at the welds on either end of the filtration layer. Differential pressures on the filtration layer, particularly when partially fouled could lead to weld failure and a bypassing of the filtration layer.
In the past, techniques for manufacturing screens for downhole use have involved formation of overlapping layers of filtration material into a tube shape and forcing the concentric layers through a die to get them joined to each other without resort to welding a layer to the nest or welding seams in each layer. This technique is illustrated in U.S. Pat. No. 5,611,399. In other applications downhole screens that were made using the welding technique to attach a filter layer to a base pipe were run in the well and expanded for open hole completions.
The present invention employs a technique of slipping the filtration layer over the base pipe and then expanding the base pipe to get a preferably interference fit with the base pipe, thereby avoiding welding in the assembly process. Different types of filtration layers are envisioned. The expansion can be over the length of the base pipe that underlays the filtration layer or even just in discrete areas of the base pipe to secure the filtration layer. These and other advantages of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment, the drawings and the claims, which all appear below.